The Third Generation Partnership Project (3GPP) has initiated the Long Term Evolution (LTE) program to bring new technology, new network architecture, new configurations and new applications and services to wireless networks in order to provide improved spectral efficiency and faster user experiences.
FIG. 1 shows an overview of an Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (E-UTRAN) 100 in accordance with the prior art. As shown in FIG. 1, E-UTRAN 100 includes three eNodeBs (eNBs) 102, however, any number of eNBs may be included in E-UTRAN 100. The eNBs 102 are interconnected by an X2 interface 108. The eNBs 102 are also connected by an S1 interface 106 to the Evolved Packet Core (EPC) 104 that includes a Mobility Management Entity (MME) 108 and a Serving Gateway (S-GW) 110.
FIG. 2 shows an LTE user-plane protocol stack 200 in accordance with the prior art. The protocol stack 200 is located in a wireless transmit receive unit (WTRU) 210 and includes the packet data control protocol (PDCP) 202, the radio link control (RLC) 204, the medium access control (MAC) 206 and the physical layer (PHY) 208. The protocol stack 200 may also reside in an eNB (not shown).
FIG. 3 shows an LTE control plane protocol stack 300 of the WTRU 210 of FIG. 2. The control plane protocol stack 300 includes the non-access stratum (NAS) 302 and a radio resource control (RRC) 304. Also included are the PDCP 306, RLC 308 and MAC 310, which together form the layer 2 sublayer 312.
Malfunction and dysfunction of the radio link, that is, the link between the WTRU and the eNB, may occur due to various reasons such as shielding, fading, interference or other mishaps due to mobility, for example. Rapid detection of a radio link failure (RLF) is important in order to initiate radio link or WTRU recovery procedures in a timely manner. Typically, RLF detection comprises downlink signal measurement performed by the physical layer entity combined with event filtering, so the WTRU may determine a course of action to follow after the problem is detected.
While performing downlink measurements, the physical layer (PHY) entity (Layer 1) of the WTRU may be configured to indicate a measurement result of “out-of-sync” or “in-sync” to the RRC entity. The WTRU is configured to count the number of out-of-sync results. The number of out-of-sync results could be counted in a counter, such as counter N310, for example. When the RRC entity counts a particular number of out-of-sync results, the RRC entity within the WTRU is configured to start a timer. For example, the WTRU may start a timer designated as the T310 timer. If the T310 timer expires prior to being stopped for another reason, the WTRU is configured to determine that an RLF has occurred.
FIG. 4 shows a discontinuous reception (DRX) cycle 400 in accordance with the prior art. Connected state DRX was introduced for LTE compliant WTRUs and eNBs for power saving purposes, in particular to conserve the battery of the WTRU. When a WTRU is in connected state DRX mode, it may “shut down” for period of time and use less power. As shown in FIG. 4, during a DRX cycle (402, 404, 406) a WTRU is able to transmit and receive during an on-duration (408, 410, 412) and does not transmit or receive during a sleep time (414, 416, 418). An eNB may be synchronized with the WTRU's DRX cycle, so that it does not send or expect to receive transmissions while the WTRU is in a sleep time.
Three parameters may be used by the WTRU and eNB to define the WTRU's DRX cycle. DRX on/off, DRX period and a non-DRX timer may be assigned values that network components can use to determine a WTRU's DRX cycle.
An LTE compliant WTRU may also be configured to operate in multiple states, where each state defines how the WTRU may be functioning in general terms. The RRC_connected state is one of the set of predefined states that a WTRU may function in. While in RRC_connected state, the WTRU may also be configured to operate in DRX mode.
When a WTRU is configured by the network to operate in DRX mode while in the RRC_connected state, the WTRU may be configured not to perform downlink measurements for RLF during the sleep time portion of the DRX cycle. The WTRU may be configured to perform the downlink radio signal measurement with respect to RLF only during the DRX on-duration and the active period.